Abstract

AIMS:

METHODS AND RESULTS:

16 pigs underwent pre-ablation T2-weighted (T2W) and late gadolinium enhancement (LGE) CMR and high-density voltage mapping of the right atrium (RA) and both were repeated after intercaval linear radiofrequency ablation. Eight pigs were sacrificed following the procedure for pathological examination. A further eight pigs were recovered for 8 weeks, before chronic CMR, repeat RA voltage mapping and pathological examination. Signal intensity (SI) thresholds from 0 to 15 SD above a reference SI were used to segment the RA in CMR images and segmentations compared with real lesion volumes. The SI thresholds that best approximated histological volumes were 2.3 SD for LGE post-ablation, 14.5 SD for T2W post-ablation and 3.3 SD for LGE chronically. T2-weighted chronically always underestimated lesion volume. Acute histology showed transmural injury with coagulative necrosis. Chronic histology showed transmural fibrous scar. The mean voltage at the centre of the ablation line was 3.3 mV pre-ablation, 0.6 mV immediately post-ablation, and 0.3 mV chronically.

CONCLUSION:

This study presents the first histopathological validation of CMR and endocardial voltage mapping to define acute and chronic atrial ablation injury, including SI thresholds that best match histological lesion volumes. An understanding of these thresholds may allow a more informed assessment of the underlying atrial substrate immediately after ablation and before repeat catheter ablation for atrial arrhythmias.

Electroanatomical mapping. Top row: CartoXPress endocardial bipolar voltage maps in a posteroanterior view (all from the same animal). Red circles indicate the site of linear radiofrequency ablation from the SVC to the IVC. Middle row: the same voltage maps have been imported in Matlab and reconstructed. Green circles indicate the site of ablation. Grey circles indicate data sampling points. Axial slices in white are shown at 4 mm intervals along the ablation line. Bottom row: graphs of endocardial voltage against distance from the centre of the ablation line (from −25 to +25 mm) for each axial slice. Red and blue lines indicate that values are ≤3 mm (non-interpolated) and ≥3 mm (interpolated) from a grey data sampling point, respectively. Only non-interpolated values were used for summary analysis in Figure .

Cardiac magnetic resonance image analysis. (A) Sagittal T2-weighted (top row) and axial late gadolinium enhancement (bottom row) cardiac magnetic resonance images. The red arrows indicate the reference region against which signal intensities were compared (left ventricular myocardium for T2-weighted and the atrial blood pool for late gadolinium enhancement images). The blue overlay indicates the region segmented as the right atrium wall (in this particular slice). These images are all pre-ablation, but the same technique was used for all image analyses. (B) A graph for one post-ablation T2-weighted image of segmentation volume against the signal intensity threshold (expressed as the number of SD above the reference signal intensity). Sample images are included to show the segmentation created (outlined in yellow) at 0, 6, and 12 SD above the reference signal intensity.

Voltage distribution across the ablation line. Average results for the 16 pre-ablation (left), 16 post-ablation (middle), and 8 chronic (right) electroanatomical maps. The solid red line indicates the mean, while the dashed red lines indicate the 95% confidence interval. Black lines indicate the raw data for all of the animals.

Segmented cardiac magnetic resonance volumes. Signal intensity thresholds from 0 to 15 standard deviations above a reference SI were applied to the right atrial wall for T2-weighted and late gadolinium enhancement images and a 3D segmentation was created for each threshold. (A and B) The results for the eight pigs, and (C and D) the results for the eight mini-pigs. Values in the tables show the statistical comparison within each graph. Dashed lines indicate ±1 SD.

Cardiac magnetic resonance thresholds. Graphs showing the comparison of cardiac magnetic resonance segmented volumes with macroscopic volumes of injury for T2-weighted (left) and late gadolinium enhancement (right). A value of >1 on the y-axis suggests that cardiac magnetic resonance is overestimating the macroscopic volume, while a value of <1 suggests underestimation. Thresholds are determined by the intersection with the line y = 1. Sample images are included to show the segmentation created (outlined in yellow) at these thresholds. Red and green dashed lines indicate ±1 SD. *, 14.5 SD, †, 2.3 SD, ‡, 3.3 SD.